The halogenated aliphatic hydrocarbon trichloroethylene (TCE) is a volatile, chlorinated compound of increasing concern as a ground water contaminant. TCE is potentially carcinogenic and is resistant to biological or abiological decomposition in subsurface waters. Conventional water treatment processes have been found to be ineffective in removing TCE from ground water. Results obtained with air stripping and adsorption to granular activated charcoal or Ambersorb resin, are more effective. These methods are present state-of-the-art technologies for remediation of VCAs. Although effective, they suffer from the fact that they simply transfer the contamination to another medium; either to the atmosphere (air stripping) or to solid medium (adsorption) which must then to disposed of as hazardous waste. The present invention results in conversion of the toxic compounds to nontoxic products.
The TCE degradation potential of microbes found in ground water systems has been examined. Evidence of anaerobic biodegration has been reported. Several studies suggest that TCE may be degraded under methanogenic conditions. However, the products of degradation include equally harmful metabolites, such as dichloroethylenes and vinyl chloride. Complete mineralization of TCE was obtained when nonsterile soils were exposed to natural gas in air, suggesting that methanotrophic microorganisms are capable of degrading TCE. A possible mechanism for the degradation of TCE by methanotrophs involves epoxidation of TCE by methane monooxygenase followed by nonbiological rearrangements that result in the formation of dichloroacetic acid, formate, and carbon monoxide, each of which would then be further degraded.
However, pure cultures of microogranisms that degrades TCE has been reported in the literature, Nelson, M.J., et al., Applied Environ. Microbiol., 53:949-954(1987) and Nelson, M.J., et al., Applied Environ. Microbiol., 54:604-606 (1988), and are the subject of copending U.S. Patent Appl. No. 44,213, the disclosures of which is hereby incorporated by reference.
These pure cultures are used in a biodegradation method that includes incubating microorganisms capable of degrading halogenated aliphatic hydrocarbons by an aromatic degradative pathway, together with halogenated aliphatic hydrocarbons under conditions such that the aromatic degradative pathway is active. The method results in the conversion of the halogenated aliphatic hydrocarbons into nontoxic products, including carbon dioxide and inorganic chloride. The method is useful in decontaminating halogenated aliphatic hydrocarbon-polluted environments. Decontaminating systems can utilize such pure cultures in bioreactor-type systems. Alternatively, natural microbial communities or introduced microorganisms can be stimulated to degrade halogenated aliphatic hydrocarbons by addition to the contaminated site of an amount of an inducer sufficient to induce an aromatic degradative pathway by which the halogenated aliphatic hydrocarbons are degraded. Unfortunately, the requirement that an inducer, e.g., methane gas or aromatic compounds, be added to activate the degradative pathway limits the bioremediation utility of these newly discovered pure cultures. Before applicant's discovery, only two types of inducers were available for biodegradation methods. Aromatic compounds, e.g., phenol, are effective inducers but their in situ use is limited by the fact they are also undesirable environmental pollutants. An alterative approach utilizes methane gas an an in situ inducer. Although effective as an inducer for the degradation of certain contaminants, the gaseous nature of methane makes it more difficult to add in situ and, also, makes controlling the biodegradation process difficult.
Accordingly, it is an object of the present invention to provide a method for biodegradation of halogenated aliphatic hydrocarbons wherein a nontoxic, nongaseous compound is utilized as an inducer to the bacterial populations or organisms. It is an additional object of the present invention to provide a method for in situ biodegradation of halogenated aliphatic hydrocarbons wherein a nontoxic, nongaseous compound is utilized as an inducer.
Further objects and advantages of the present invention will be apparent from the discussion that follows.